(1) Field of the Invention
This invention concerns bond-ply materials comprising at least one resin and at least one nonwoven reinforcing material. The present invention also includes methods for using the bond-ply materials of this invention to manufacture high density multilayer printed wiring boards.
(2) Description of the Art
The electronics industry continues to seek enhanced product performance in order to meet consumer demands for higher functionality and lower cost computers and other electronic devices. Among the methods that the electronic industry is using to increase performance is by designing of multilayer circuit boards that have smaller, finer circuit lines and spaces. Increasing line density leads to fewer circuit layers resulting in smaller electronic devices. The methods for fabrication of multilayer boards are well-known in the art. One commonly used method is to use a joint layer to unite two separate circuit layers. Such joint layer is known in the art as prepreg or bond-ply. Through holes or vias are drilled in the bond-ply and metallized to form electrical interconnections between the circuit layers.
Recently, the industry began migrating towards micro-via technologies to join and electrically interconnect multiple circuit layers. The use of micro-vias, typically smaller than 150 microns in diameter, significantly densifies the circuitry due to reduced amount of board area needed per via. Lasers are used in the printed wiring board industry to drill micro-vias. The methods for using lasers to drill micro-vias in electronic substrates such as films and nonwoven reinforced laminates have been described in the prior art.
The configuration and properties of a bond-ply material significantly affect the fabrication and performance of a multilayer printed wiring board. With increasing circuit density and finer circuit traces it is becoming more important to use bond-plys in the fabrication of high density multilayer wiring boards that have good dimensional stability and that behave uniformly when subjected to environmental stress over dimensions comparable to or smaller than the dimensions of the circuitry to facilitate registration of the vias in the bond-ply with the pads in the circuit layers. Furthermore dimensional uniformity of the bond-ply is also required in order to assure that warping or buckling does not occur after processing and circuit fabrication and assembly. In general it is known that fiber reinforcement moderate of the relatively high thermal coefficient of expansion of polymeric materials, that it provides for mechanical rigidity, and that fiber reinforcement can reduce the brittleness of composite materials. It is also important that the bond-ply can be consistently laser drilled to form micro-vias as mechanical drilling of microvias is inefficient and expensive. Furthermore, it has become important to have a uniform thickness of dielectric material between the circuit layers that are joined by a bond-ply. In general, however, prior art reinforced bond-ply materials are either not sufficiently uniform to prevent warp and buckle or cannot be laser drilled to form uniform micro vias. Prior art reinforced bond-ply materials also are generally insufficiently uniform to provide a consistent and known final dielectric thickness between circuit layers after joining them.
Despite the advancements made in the quality of electronic substrates, there remains a need for improvements. Specifically, there remains a need for a bond-ply that cleanly ablate when lased to produce quality through holes or vias. There also remains a need for reinforced bond-plies with improved dimensional stability and strength. And there remains a need for bond-plies that can be efficiently and effectively used to manufacture high density multilayer wiring boards. There also remains a need for a bond-ply that provides for a uniform and known dielectric thickness between two circuit layers after joining the layers together.